Epoxy resin composition, prepreg, and fiber-reinforced composite material

ABSTRACT

The present invention provides an epoxy resin composition that serves to produce a cured epoxy resin that simultaneously realizes a high heat resistance, a high elastic modulus, and a low color and to produce a molded article having a good appearance without suffering the formation of white spots on the surface thereof when used as matrix resin in a fiber reinforced composite material. The epoxy resin composition includes an epoxy resin as component [A] and an imidazole compound as component [B] and meets certain conditions (a) to (d).

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2017/023188, filedJun. 23, 2017, which claims priority to Japanese Patent Application No.2016-127268, filed Jun. 28, 2016, the disclosures of these applicationsbeing incorporated herein by reference in their entireties for allpurposes.

FIELD OF THE INVENTION

The present invention relates to an epoxy resin composition adapted foruse as matrix resin of a fiber reinforced composite material suitablefor sport applications and general industry applications, and alsorelates to a prepreg and a fiber reinforced composite material preparedby using such an epoxy resin composition as matrix resin.

BACKGROUND OF THE INVENTION

Epoxy resins have been widely used in various industries as a coatingcomposition, adhesive, electric and electronic information material,advanced composite material, or the like because of their excellentmechanical properties. Epoxy resins have been frequently usedparticularly in fiber reinforced composite materials composed mainly ofa reinforcing fiber, such as carbon fiber, glass fiber, and aramidfiber, combined with a matrix resin.

The use of a prepreg produced by impregnating a carbon fiber base withan epoxy resin is popular in the production of carbon fiber reinforcedcomposite materials. Such a prepreg is laminated in layers or preformed,and then heated to cure the epoxy resin, thereby producing a moldedarticle. If the curing reaction proceeds before the lamination step, theprepreg will suffer deterioration in handling property. Therefore, anepoxy resin to use in a prepreg requires high preservation stability,and dicyandiamide is widely used as curing agent because it is high inlatent curing property.

Being lightweight and having high strength and high stiffness, carbonfiber composite materials have been used in a variety of fields rangingfrom sport and leisure goods to industrial applications such asautomobiles and aircraft. With this feature, they have been frequentlyused in recent years not only in structure members, but fortexture-of-cloth decoration realized by arranging woven fabrics insurfaces. For an epoxy resin adopted as matrix resin, therefore,importance is now attached to the low colors of cured products andappearance of molded articles in addition to high heat resistance andgood mechanical properties of cured products. If dicyandiamide is usedas hardener, however, there occurs the problem of white spots beingformed on the surface of the resulting molded article, leading todeterioration in the appearance thereof.

As a method to prevent the formation of white spots attributed todicyandiamide, Patent document 1 discloses a technique designed todepress the formation of white spots in prepreg by adopting amasterbatch containing dicyandiamide particles with small diameters toallow the dicyandiamide and epoxy resin to be dissolved orcompatibilized during the step for impregnating the base. As methods toeliminate the use of dicyandiamide, Patent document 2 discloses atechnique that uses polythiol and a urea compound as hardener componentsand Patent document 3 discloses a technique that employs an acidanhydride as hardener.

PATENT DOCUMENTS

-   Patent document 1: Japanese Unexamined Patent Publication (Kokai)    No. HEI 11-209580-   Patent document 2: Japanese Unexamined Patent Publication (Kokai)    No. 2013-253194-   Patent document 3: Japanese Unexamined Patent Publication (Kokai)    No. 2013-133407

SUMMARY OF THE INVENTION

In the case of the method described in Patent document 1, however,dicyandiamide is dissolved or compatibilized during the prepregproduction step and as a result, the prepreg obtained fails to realize asufficiently high preservation stability when used in a fiber reinforcedcomposite material. In the case where dicyandiamide is not dissolved inthe prepreg production step, the resulting molded articles occasionallysuffer the formation of white spots on the surfaces thereof.

In the case of the method described in Patent document 2, the resultingmolded articles do not suffer the formation of white spots on thesurfaces thereof because dicyandiamide is not present, but the resultingcured resin products occasionally fail to have sufficiently high heatresistance or good mechanical properties.

In the case of using an acid anhydride hardener as proposed in Patentdocument 3, the resulting molded articles are free of white spots on thesurfaces thereof, but the acid anhydride used as the hardener isoccasionally degraded by the moisture in air, possibly leading to acured resin product with deteriorated physical properties. Accordingly,it is not preferred to apply this method to the production of a prepregfor a fiber reinforced composite material that is expected to be storedfor a certain period.

An object of the present invention is to eliminate the drawbacks ofthese conventional techniques to provide an epoxy resin composition thatserves to produce a cured epoxy resin simultaneously realizing a highheat resistance, a high elastic modulus, and a low color and to producea molded article having a good appearance without suffering theformation of white spots on the surface thereof when used as matrixresin in a fiber reinforced composite material, and also provide aprepreg produced from the epoxy resin composition, and a fiberreinforced composite material that is obtained by curing the prepreg andthat suffers no white spots on the surface thereof and has a goodappearance.

After making an intensive study aiming to solve the problems describedabove, the inventors of the present invention found an epoxy resincomposition having the following constitution and arrived at the presentinvention on the basis of the finding. More specifically, the epoxyresin composition according to the present invention has theconstitution described below.

An epoxy resin composition containing epoxy resins as component [A] andan imidazole compound as component [B] and meeting the undermentionedconditions (a) to (d):

(a) it contains an isocyanurate type epoxy resin [A1] as component [A]in an amount of 10 to 40 parts by mass relative to the total quantity ofepoxy resins which represents 100 parts by mass,

(b) it contains an bisphenol type epoxy resin [A2] as a component [A] inan amount of 40 to 90 parts by mass relative to the total quantity ofepoxy resins which represents 100 parts by mass,

(c) resin [A2] has an average epoxy equivalent weight of 220 to 500g/eq, and

(d) the content of component [B] is such that the ratio of the number ofimidazole groups to the total number of epoxy groups in the epoxy resinsis 0.01 to 0.06.

The prepreg according to the present invention is a prepreg thatincludes the aforementioned epoxy resin composition and reinforcingfiber.

Furthermore, the fiber reinforced composite material according to thepresent invention is a fiber reinforced composite material that isobtainable by curing the aforementioned prepreg.

The present invention can provide an epoxy resin composition that servesto produce a cured epoxy resin realizing a high heat resistance, goodmechanical properties and a low color and to produce a molded articlehaving a good appearance without suffering the formation of white spotson the surface thereof when used as matrix resin in a fiber reinforcedcomposite material.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The epoxy resin composition according to the present invention containsan epoxy resin as component [A] and an imidazole compound as component[B], both being essential components.

<Component [A]>

Component [A] for the present invention consists of epoxy resins.Examples thereof include glycidyl ether type epoxy resins such asbisphenol type epoxy resins, biphenyl type epoxy resins, naphthalenetype epoxy resins, novolac type epoxy resins, epoxy resins havingfluorene backbones, epoxy resins formed from copolymers of a phenolcompound and dicyclopentadiene, diglycidyl resorcinol,tetrakis(glycidyloxyphenyl)ethane, and tris(glycidyloxyphenyl)methane;and glycidylamine type epoxy resins such as tetraglycidyldiaminodiphenylmethane, triglycidyl aminophenol, triglycidylaminocresol, and tetraglycidyl xylene diamine.

For the present invention, an isocyanurate type epoxy resin [A1] iscontained as component [A].

As component [A1] is contained, it serves to produce a cured resinproduct having a high elastic modulus and also having an improved heatresistance, thus allowing the production of a fiber reinforced compositematerial having good mechanical properties and a high heat resistance.

It is essential for component [A1] to account for 10 to 40 parts by massrelative to the total quantity of epoxy resins, which accounts for 100parts by mass, in the epoxy resin composition, and it is preferable thatthe lower limit is 15 parts by mass or more and that the upper limit is30 parts by mass or less. As component [A1] is contained in an amount inthis range, it serves to produce a cured resin product having a lowcolor and also having an elastic modulus and a heat resistance in a goodbalance.

Examples of useful commercial products of component [A1] include TEPIC(registered trademark) -S, -L, -PAS B22 (all manufactured by NissanChemical Industries, Ltd.) and Araldite (registered trademark) PT9810(manufactured by Huntsman Advanced Materials Gmbh).

For the present invention, a bisphenol type epoxy resin [A2] iscontained as component [A]. As resin [A2] is contained, it serves toproduce a cured resin product having a decreased color and a fiberreinforced composite material having a good appearance.

It is essential for resin [A2] to account for 40 to 90 parts by massrelative to the total quantity of epoxy resins, which accounts for 100parts by mass, in the epoxy resin composition, and it is preferable thatthe lower limit is 70 parts by mass or more and that the upper limit is90 parts by mass or less. If resin [A2] is contained in an amount inthis range, it serves to produce a cured resin product having a colorand an elastic modulus in a good balance.

Examples of resin [A2] include epoxy resins produced byglycidyl-etherification of bisphenol compounds such as bisphenol A typeepoxy resin, bisphenol F type epoxy resin, bisphenol AD type epoxyresin, and bisphenol S type epoxy resin.

Examples of useful commercial products of bisphenol A type epoxy resininclude jER (registered trademark) 825, 828, 834, 1001, 1002, 1003,1003F, 1004, 1004AF, 1005F, 1006FS, 1007, 1009, and 1010 (allmanufactured by Mitsubishi Chemical Corporation).

Examples of useful commercial products of bisphenol F type epoxy resininclude jER (registered trademark) 806, 807, 4002P, 4004P, 4007P, 4009P,and 4010P (all manufactured by Mitsubishi Chemical Corporation),Epotohto (registered trademark) YDF2001 and YDF2004 (both manufacturedby Nippon Steel Chemical Co., Ltd.), and EPICRON (registered trademark)830, 830-S, and 835 (all manufactured by DIC Corporation).

Examples of useful commercial products of bisphenol S type epoxy resininclude EPICRON (registered trademark) EXA-1514 (manufactured by DICCorporation).

For the present invention, it is essential for resin [A2] in the epoxyresin composition to have an average epoxy equivalent weight of 220 to500 g/eq from the viewpoint of the balance between the heat resistanceand the color of the cured epoxy resin and it is preferable that thelower limit is 300 g/eq or more and that the upper limit is 400 g/eq orless. If the average epoxy equivalent weight of resin [A2] is less than220 g/eq, the resulting cured resin will have a decreased heatresistance and increased coloring, possibly leading to a fiberreinforced composite material with a poor appearance. On the other hand,if the average epoxy equivalent weight of resin [A2] is more than 500g/eq, the coloring will be weak, but the resulting cured resin will havea decreased heat resistance.

The average epoxy equivalent weight of resin [A2] in the epoxy resincomposition can be calculated by the method described below.

(Method for Calculation of the Average Epoxy Equivalent Weight of Resin[A2] in the Epoxy Resin Composition)

In the case of an epoxy resin composition containing a combination of ntypes of epoxy resins as resin [A2], with the epoxy resins in resin [A2]accounting for G′ parts by mass in total and each epoxy resin X, whichhas an epoxy equivalent weight of Ex (g/eq), in resin [A2] accountingfor Wx parts by mass, the average epoxy equivalent weight (g/eq) ofresin [A2] is calculated by Equation (I) (where x=1, 2, 3, . . . , n)given below.

$\begin{matrix}{\mspace{79mu}\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack} & \; \\{{{Average}\mspace{14mu}{epoxy}\mspace{14mu}{equivalent}\mspace{14mu}{weight}\mspace{14mu}{of}\mspace{14mu}{{resin}\mspace{14mu}\left\lbrack {A\; 2} \right\rbrack}\mspace{14mu}{in}\mspace{14mu}{epoxy}\mspace{14mu}{resin}\mspace{14mu}{composition}\mspace{14mu}\left( {g\text{/}{eq}} \right)} = \frac{G^{\prime}}{\left\{ {\left( \frac{W_{1}}{E_{1}} \right) + {\left( \frac{W_{2}}{E_{2}} \right)\ldots} + {\left( \frac{W_{x}}{E_{x}} \right)\ldots} + \left( \frac{W_{n}}{E_{n}} \right)} \right\}}} & (I)\end{matrix}$

<Component [B]>

For the present invention, component [B] is an imidazole compound. Forthe present invention, the imidazole compound in component [B] works asa hardener to promote the self-polymerization of the epoxy resins incomponent [A]. The use of an imidazole compound serves to produce acured epoxy resin having a low color and a good balance with heatresistance in comparison with other self-polymerization type hardeners.

For the present invention, the content of component [B] is such that theratio of the number of imidazole groups to the total number of epoxygroups in the epoxy resins (i.e., the molar ratio of imidazole to theepoxy groups in all epoxy resins) is 0.01 to 0.06 from the viewpoint ofthe balance between the heat resistance and the color of the cured epoxyresin, and it is preferable that the lower limit is 0.015 or more andthat the upper limit is 0.05 or less. If the content of component [B] isso small that the ratio of the number of imidazole groups to the numberof epoxy groups is less than 0.01, it leads to a cured resin with adecreased heat resistance. If the content of component [B] is so largethat the ratio of the number of imidazole groups to the number of epoxygroups is more than 0.06, it leads to a cured resin with a high colorand accordingly leads to a fiber reinforced composite material with apoor appearance.

The aforementioned ratio of the number of imidazole groups to the numberof epoxy groups is calculated by the method described below.

(Method for Calculation of Ratio of the Number of Imidazole Groups tothe Number of Epoxy Groups in Epoxy Resin Composition)

(1) Calculation of average epoxy equivalent weight over all epoxy resins

In the case of an epoxy resin composition containing a combination of ntypes of epoxy resins as component [A], with the epoxy resins accountingfor G parts by mass in total and each epoxy resin Y, which has an epoxyequivalent weight of Ey (g/eq), accounting for Wy parts by mass, theaverage epoxy equivalent weight (g/eq) over all epoxy resins iscalculated by Equation (II) (where y=1, 2, 3, . . . , n) given below.

$\begin{matrix}{\mspace{79mu}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack} & \; \\{{{Average}\mspace{14mu}{epoxy}\mspace{14mu}{equivalent}\mspace{14mu}{weight}\mspace{14mu}{over}\mspace{14mu}{all}\mspace{14mu}{epoxy}\mspace{14mu}{resins}\mspace{14mu}{in}\mspace{14mu}{epoxy}\mspace{14mu}{resin}\mspace{14mu}{composition}\mspace{14mu}\left( {g\text{/}{eq}} \right)} = \frac{G}{\left\{ {\left( \frac{W_{1}}{E_{1}} \right) + {\left( \frac{W_{2}}{E_{2}} \right)\ldots} + {\left( \frac{W_{x}}{E_{x}} \right)\ldots} + \left( \frac{W_{n}}{E_{n}} \right)} \right\}}} & ({II})\end{matrix}$

(2) Calculation of the ratio of the number of imidazole groups to thenumber of epoxy groups In the case of an epoxy resin compositioncontaining epoxy resins with a total quantity accounting for G parts bymass and also containing an imidazole compound accounting for W parts bymass in the epoxy resin composition and having an imidazole equivalentweight of I (g/eq), the ratio of the number of imidazole groups to thenumber of epoxy groups in the epoxy resin composition is calculated byEquation (III) from the value obtained in the above paragraph (1).[Formula 3]Ratio of the number of imidazole groups to the number of epoxy groups inepoxy resin composition=average epoxy equivalent weight of epoxy resincomposition×W/(G×I)  (III)

Examples of the imidazole compounds include 1-benzyl-2-methyl imidazole,1-benzyl-2-ethyl imidazole, 1-cyanoethyl-2-methyl imidazole,1-cyanoethyl-2-ethyl-4-methyl imidazole, and 1-cyanoethyl-2-phenylimidazole. These imidazole compounds may be used singly or as acombination of two or more thereof. When a plurality of imidazolecompounds are used in combination, the ratio of the number of imidazolegroups to the number of epoxy groups is determined by calculating theaverage imidazole equivalent weight as in the case of the average epoxyequivalent weight and using it as the aforementioned imidazoleequivalent weight. Hereinafter, both the imidazole equivalent weight forthe case where an imidazole compound is used singly and the averageimidazole equivalent weight for the case where a plurality of imidazolecompounds are used in combination are referred to collectively as theimidazole equivalent weight of component [B].

It is preferable for component [B] to have an imidazole equivalentweight of 180 g/eq or more. If component [B] has an imidazole equivalentweight of 180 g/eq or more, it will tend to result in a cured epoxyresin with a reduced color and an improved heat resistance, making itpossible to easily produce a fiber reinforced composite material havinga good appearance and a high heat resistance. The upper limit of theimidazole equivalent weight is preferably 1,000 g/eq or less.

From the viewpoint of increasing the imidazole equivalent weight, it ispreferable that a compound [B1] as described by the undermentionedgeneral formula (I) is contained as component [B].

(In the formula, R₁, R₂, R₃ and R₄ are each independently a hydrogenatom, an aliphatic hydrocarbon group with a carbon number of 1 to 20, ora phenyl group, and X is an alkylene group or an aromatic hydrocarbongroup.)

Compound [B1] is an addition product obtainable through a reactionbetween an imidazole compound and an isocyanate compound. Commercialproducts of the addition product include G-8009L (manufactured by DKSCo. Ltd.).

Similarly, from the viewpoint of increasing the imidazole equivalentweight, furthermore, it is also preferable that a compound [B2] asrepresented by the undermentioned general formula (II) is contained ascomponent [B].

(In the formula, R₅, R₆, R₇, and R₈ are each independently a hydrogenatom, an aliphatic hydrocarbon group with a carbon number of 1 to 20, ora phenyl group, and Y is a single bond, an alkylene group, an alkylidenegroup, an ether group, or a sulfonyl group).

Compound [B2] is an addition product obtainable through a reactionbetween an imidazole compound and an epoxy compound. Commercial productsof the addition product include Cureduct (registered trademark) P-0505(Shikoku Chemicals Corporation) and JER cure (registered trademark)P200H50 (Mitsubishi Chemical Corporation).

The incorporation of a dicyandiamide as hardener may result in a moldedarticle suffering the formation of white spots on the surface thereof todeteriorate the appearance. For the present invention, therefore, it ispreferable for the dicyandiamide to account for 0.5 part by mass orless, more preferably 0.2 part by mass or less, relative to the totalquantity of epoxy resins, which accounts for 100 parts by mass, and itis most preferable that dicyandiamide is not contained.

<Component [C]>

An acidic compound may be added to as component [C] the epoxy resincomposition according to the present invention. For the presentinvention, the acidic compound of component [C] works as a stabilizationagent for the imidazole compound of component [B]. The addition of anacidic compound is preferred in order to provide an epoxy resincomposition or prepreg with an improved preservation stability.

Examples of the acidic compound include Broensted acid and Lewis acid.

Preferred examples of the Broensted acid include various carboxylicacids. The carboxylic acids can be categorized into aliphaticmonocarboxylic acids, aromatic monocarboxylic acids, aliphaticpolycarboxylic acids, and aromatic polycarboxylic acids, and include,for example, the following compounds.

Examples of aliphatic monocarboxylic acids include formic acid, aceticacid, propionic acid, butyric acid, isobutyric acid, valeric acid,caproic acid, enanthic acid, caprylic acid, octyl acid, pelargonic acid,lauryl acid, myristic acid, stearic acid, behenic acid, undecane acid,acrylic acid, methacrylic acid, crotonic acid, oleic acid, andderivatives thereof.

Examples of aliphatic polycarboxylic acids include oxalic acid, malonicacid, succinic acid, glutaric acid, adipic acid, pimelic acid, subericacid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioicacid, tridecanedioic acid, tetradecanedioic acid, pentadecanedioic acid,and derivatives thereof.

Examples of aromatic monocarboxylic acids include benzoic acid, cinnamicacid, naphthoic acid, toluic acid, and derivatives thereof.

Examples of aromatic polycarboxylic acids include phthalic acid,isophthalic acid, terephthalic acid, trimellitic acid, pyromelliticacid, and derivatives thereof.

Of these Broensted acids, it is preferable to use an aromaticmonocarboxylic acid, particularly preferably benzoic acid, as compound[C1].

As a Lewis acid, the use of boric acid and/or a borate ester compound orthe like is desirable.

Examples of the boric acid and/or a borate ester compound include boricacid; alkyl borates such as trimethyl borate, triethyl borate, tributylborate, tri-n-octyl borate, tri(triethylene glycol methyl ether) borate,tricyclohexyl borate, and trimethyl borate; aromatic borates such astri-o-cresyl borate, tri-m-cresyl borate, tri-p-cresyl borate, andtriphenyl borate; and others such as tri(1,3-butanediol) biborate,tri(2-methyl-2,4-pentanediol) biborate, and trioctylene glycol diborate.

The boric ester compound to use may also be a cyclic boric estercompound having a cyclic structure in its molecule. Examples of thecyclic boric ester compound include tris-o-phenylene bisborate,bis-o-phenylene pyroborate, bis-2,3-dimethylethylene pyroborate, andbis-2,2-dimethyltrimethylene pyroborate.

Of these Lewis acids, the use of a borate ester compound as compound[C2] is particularly preferable. Commercial products that contain suchboric ester compounds include Cureduct (registered trademark) L-01B andL-07N (both manufactured by Shikoku Chemicals Corporation).

From the viewpoint of the balance between the preservation stability ofthe resin composition and the heat resistance, mechanical properties,and color of the resulting cured epoxy resin, there must exist anappropriate combination of an imidazole compound of component [B] and anacidic compound of component [C]. In particular, it is preferable to usea benzoic acid [C1] for compound [B1] and a borate ester compound [C2]for compound [B2].

<Method for Preparation of Epoxy Resin Composition>

To prepare the epoxy resin composition according to the presentinvention, a mixture of materials may be kneaded by using a machine suchas kneader, planetary mixer, three roll mill, and twin screw extruder,or a mixture may be manually produced by using, for example, a beakerand a spatula if uniform kneading is possible. Preferred preparationmethods include the following. Specifically, component [A] is put in acontainer and heated while stirring to an appropriate temperature in therange of 130° C. to 180° C. to ensure uniform dissolution of the epoxyresin. Subsequently, it is cooled while stirring preferably to atemperature of 100° C. or less, more preferably 80° C. or less, andstill more preferably 60° C. or less, and component [B] and component[C] are fed, followed by kneading. In this instance, it is morepreferred to prepare in advance a curing agent masterbatch using part ofcomponent [A] in order to ensure uniform mixing of component [B] andcomponent [C].

<Fiber Reinforced Composite Material>

Next, details of the fiber reinforced composite material are describedbelow. A fiber reinforced composite material containing a cured productof the epoxy resin composition according to the present invention asmatrix resin can be produced by blending and integrating the epoxy resincomposition according to the present invention with reinforcing fiber,followed by curing the blend

There are no specific limitations on the reinforcing fiber to be usedfor the present invention, and useful examples include glass fiber,carbon fiber, aramid fiber, boron fiber, alumina fiber, and siliconcarbide fiber. A plurality of these fibers may be used as a mixture. Ofthese, the use of carbon fiber is preferred because it serves to providea fiber reinforced composite material that is light in weight and highin rigidity and a molded article containing glossy black fibers andhaving good design characteristics.

The white spots on the surface of a molded article that occur whendicyandiamide is used as hardener, which constitute the major problem ofthe present invention, are considered to result from the filtering ofdicyandiamide by fibers during the impregnation with the epoxy resincomposition or the segregation of dicyandiamide in the neighborhood offibers as the resin flows during the molding step. When dicyandiamide isused as hardener, the advantageous effect of the invention is enhancedmore greatly with a decreasing single fiber diameter of the reinforcingfiber that is combined with the fiber reinforced composite materialbecause fibers with a smaller single fiber diameter tend to suffer morewhite spots. From this viewpoint, it is preferable for the reinforcingfiber to have a single fiber diameter of 3 to 20 μm, more preferably 3to 10 μm.

<Prepreg>

When producing a fiber reinforced composite material, it is preferableto preliminarily produce a prepreg composed mainly of an epoxy resincomposition and reinforcing fiber in order to ensure easy storage andhigh handleability. Such a prepreg can be obtained by impregnatingreinforcing fiber with the epoxy resin composition according to thepresent invention. Good techniques for the impregnation include hotmelting (dry method).

Hot melting is a technique designed for direct impregnation ofreinforcing fiber with an epoxy resin composition that is preliminarilyheated to decrease its viscosity. Specifically, a film coated with anepoxy resin composition is first prepared on a piece of release paper orthe like, and then the film is put on a sheet of paralleled reinforcingfibers or a sheet (cloth) of woven fabric reinforcing fibers from bothsides or from one side thereof, and heated and pressed to ensureimpregnation of the reinforcing fiber with the resin.

There are no specific limitations on the structural features of thereinforcing fiber to use for producing the prepreg, but it is preferredto use woven fabrics because they serve to produce molded articles withbeautiful weave patterns and good design characteristics. In the casewhere dicyandiamide is used as hardener, white spots tend to begenerated in the neighborhood of intersections of fibers as a wovenfabric type prepreg is molded. The advantageous effect of the inventionis enhanced particularly greatly when woven fabric is used asreinforcing fiber in a prepreg.

Here, there are no specific limitations on the reinforcing fiber to usein a prepreg, and the various fibers listed above in the description offiber reinforced composite materials can be adopted. Among others, theuse of carbon fiber is preferred because it serves to provide a fiberreinforced composite material that is light in weight and high inrigidity and a molded article containing glossy black fibers and havinggood design characteristics.

<Molding Method for Prepreg>

For laminate molding of prepreg layers, techniques that can be usefulfor applying heat and pressure include press molding, autoclave molding,bucking molding, wrapping tape molding, and internal pressure molding.

Fiber reinforced composite materials containing a cured product of theepoxy resin composition according to the present invention andreinforcing fiber are adopted favorably in sports applications, generalindustrial applications, and aerospace applications. More specifically,preferred sports applications include golf shafts, fishing rods, tennisand badminton rackets, hockey and other sticks, and skiing poles.Furthermore, preferred general industrial applications includestructural and interior finishing material of vehicles (such asautomobiles, motorcycles, bicycles, ships, and railroad vehicles), driveshafts, plate springs, windmill blades, pressure vessels, flywheels,rollers for paper manufacture, roofing materials, cables, andmending/reinforcing materials.

EXAMPLES

The present invention is described below in more detail with referenceto Examples, but it should be understood that the invention is notconstrued as being limited thereto.

Determination of physical properties was performed in an environmentwith a temperature of 23° C. and a relative humidity of 50% unlessotherwise specified.

The materials listed below were used for the preparation of variousepoxy resin compositions.

<Materials Used>

Component [A]: epoxy resin

-   -   isocyanurate type epoxy resin [A1]

[A1]-1 TEPIC (registered trademark) -S (epoxy equivalent weight 100,manufactured by Nissan Chemical Industries, Ltd.)

[A1]-2 TEPIC (registered trademark) -L (epoxy equivalent weight 101,manufactured by Nissan Chemical Industries, Ltd.)

[A1]-3 TEPIC (registered trademark) -PAS B22 (epoxy equivalent weight190, manufactured by Nissan Chemical Industries, Ltd.).

-   -   bisphenol type epoxy resin [A2]

[A2]-1 EPICLON (registered trademark) 830 (bisphenol F type epoxy resin,epoxy equivalent weight 172, manufactured by DIC Corporation)

[A2]-2 jER (registered trademark) 828 (bisphenol A type epoxy resin,epoxy equivalent weight 189, manufactured by Mitsubishi ChemicalCorporation)

[A2]-3 Epotohto (registered trademark) YDF-2001 (bisphenol F type epoxyresin, epoxy equivalent weight 475, manufactured by Tohto Kasei Co.,Ltd.)

[A2]-4 jER (registered trademark) 1001 (bisphenol A type epoxy resin,epoxy equivalent weight 470, manufactured by Mitsubishi ChemicalCorporation)

[A2]-5 jER (registered trademark) 4004P (bisphenol F type epoxy resin,epoxy equivalent weight 910, manufactured by Mitsubishi ChemicalCorporation)

[A2]-6 jER (registered trademark) 1007 (bisphenol A type epoxy resin,epoxy equivalent weight 910, manufactured by Mitsubishi ChemicalCorporation)

-   -   other epoxy resins [A3]

[A3]-1 SUMI-EPDXY (registered trademark) ELM434 (diaminodiphenylmethanetype epoxy resin, epoxy equivalent weight 120, manufactured by SumitomoChemical Co., Ltd.)

[A3]-2 jER (registered trademark) 154 (phenol novolac type epoxy resin,epoxy equivalent weight 175, manufactured by Mitsubishi ChemicalCorporation)

Component [B]: Imidazole Compound

-   -   compound [B1] as represented by general formula (I)

[B1]-1 G-8009L (imidazole equivalent weight 195, compound as representedby general formula (I) where R₁ and R₂ are each an ethyl group, R₃ andR₄ are each a methyl group, and X is a hexamethylene group, manufacturedby Dai-Ichi Kogyo Seiyaku Co., Ltd.)

-   -   compound [B2] as represented by general formula (II)

[B2]-1 Cureduct (registered trademark) P-0505 (imidazole equivalentweight 280, compound as represented by general formula (II) where R₅ andR₆ are each an ethyl group, R₇ and R₈ are each an methyl group, and Y isan isopropylidene group, manufactured by Shikoku Chemicals Corporationindustry)

-   -   other imidazole compounds [B3]

[B3]-1 Curezol (registered trademark) 2MZ-H (imidazole equivalent weight82, 2-methyl imidazole, manufactured by Shikoku Chemicals Corporationindustry)

[B3]-2 Curezol (registered trademark) 2PZ (imidazole equivalent weight144, 2-phenyl imidazole, manufactured by Shikoku Chemicals Corporationindustry)

-   -   [B′] hardener other than imidazole compound

[B′]-1 jER Cure (registered trademark) DICY7 (dicyandiamide,manufactured by Mitsubishi Chemical Corporation)

[B′]-2 DCMU99 (3-(3,4-dichlorophenyl)-1,1-dimethylurea, manufactured byHodogaya Chemical Co., Ltd.)

Component [C]: acidic compound

-   -   [C1] aromatic carboxylic acid

[C1]-1 benzoic acid (manufactured by Tokyo Chemical Industry Co., Ltd.)

-   -   [C2] a mixture containing a borate ester compound

[C2]-1 Curezol (registered trademark) L-01B (a mixture containing 5parts by mass of a borate ester compound as acidic compound,manufactured by Shikoku Chemicals Corporation industry)

[C2]-2 Curezol (registered trademark) L-07N (a mixture containing 5parts by mass of a borate ester compound as acidic compound,manufactured by Shikoku Chemicals Corporation industry)

<Methods for Calculation of Parameters of Resin Composition>

(1) Method for Calculation of the Average Epoxy Equivalent Weight ofResin [A2] in an Epoxy Resin Composition

In the case of an epoxy resin composition containing n types of epoxyresins in combination as resin [A2], with the epoxy resins in resin [A2]accounting for G′ parts by mass in total and each epoxy resin X, whichhas an epoxy equivalent weight of Ex (g/eq), in resin [A2] accountingfor Wx parts by mass, the average epoxy equivalent weight (g/eq) ofresin [A2] was calculated by Equation (I) (where x=1, 2, 3, . . . , n)given below.

$\begin{matrix}{\mspace{79mu}\left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack} & \; \\{{{Average}\mspace{14mu}{epoxy}\mspace{14mu}{equivalent}\mspace{14mu}{weight}\mspace{14mu}{of}\mspace{14mu}{{resin}\mspace{14mu}\left\lbrack {A\; 2} \right\rbrack}\mspace{14mu}{in}\mspace{14mu}{epoxy}\mspace{14mu}{resin}\mspace{14mu}{composition}\mspace{14mu}\left( {g\text{/}{eq}} \right)} = \frac{G^{\prime}}{\left\{ {\left( \frac{W_{1}}{E_{1}} \right) + {\left( \frac{W_{2}}{E_{2}} \right)\ldots} + {\left( \frac{W_{x}}{E_{x}} \right)\ldots} + \left( \frac{W_{n}}{E_{n}} \right)} \right\}}} & (I)\end{matrix}$

(2) Method for Calculation of Average Epoxy Equivalent Weight Over allEpoxy Resins in Epoxy Resin Composition

In the case of an epoxy resin composition containing a combination of ntypes of epoxy resins as component [A] and also containing epoxy resinsaccounting for G parts by mass in total, with each epoxy resin Y, whichhas an epoxy equivalent weight of Ey (g/eq), accounting for Wy parts bymass, the average epoxy equivalent weight (g/eq) over all epoxy resinsis calculated by Equation (II) (where y=1, 2, 3, . . . , n) given below.

$\begin{matrix}{\mspace{79mu}\left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack} & \; \\{{{Average}\mspace{14mu}{epoxy}\mspace{14mu}{equivalent}\mspace{14mu}{weight}\mspace{14mu}{over}\mspace{14mu}{all}\mspace{14mu}{epoxy}\mspace{14mu}{resins}\mspace{14mu}{in}\mspace{14mu}{epoxy}\mspace{14mu}{resin}\mspace{14mu}{composition}\mspace{14mu}\left( {g\text{/}{eq}} \right)} = \frac{G}{\left\{ {\left( \frac{W_{1}}{E_{1}} \right) + {\left( \frac{W_{2}}{E_{2}} \right)\ldots} + {\left( \frac{W_{x}}{E_{x}} \right)\ldots} + \left( \frac{W_{n}}{E_{n}} \right)} \right\}}} & ({II})\end{matrix}$

(3) Method for Calculation of Ratio of the Number of Imidazole Groups tothe Number of Epoxy Groups in Epoxy Resin Composition

In the case of an epoxy resin composition containing epoxy resins with atotal quantity accounting for G parts by mass and also containing animidazole compound accounting for W parts by mass in the epoxy resincomposition and having an imidazole equivalent weight of I (g/eq), theratio of the number of imidazole groups to the number of epoxy groups inthe epoxy resin composition was calculated by Equation (III) from thevalue obtained in the above paragraph (2).[Formula 6]Ratio of the number of imidazole groups to the number of epoxy groups inepoxy resin composition=average epoxy equivalent weight of epoxy resincomposition×W/(G×I)  (III)

<Method for Preparation of Epoxy Resin Composition>

(1) Preparation of Curing Agent Masterbatch

A liquid-state bisphenol type epoxy resin [A2] ([A2]-1 and/or [A2]-2 tobe contained in the resin composition) was prepared in an amount of 10parts by mass (accounting for 10 parts by mass relative to the totalquantity of epoxy resins in component [A], which accounts for 100 partsby mass). To this, an imidazole compound for component [B], a hardener[B′] other than the imidazole compound, and an acidic compound forcomponent [C] that are to be contained in the resin composition areadded and kneaded in a kneader at room temperature. The resultingmixture was passed through a three roll mill twice to prepare a curingagent masterbatch.

(2) Preparation of Epoxy Resin Composition

Excluding the 10 parts by mass of liquid-state bisphenol type epoxyresin [A2] used in paragraph (1) above, 90 parts by mass of the epoxyresin of component [A] is put in a kneader. While kneading, thetemperature was raised to 150° C. and it was maintained for 1 hour toprovide a transparent viscous liquid. After cooling the liquid to 60° C.while continuing to knead it, the curing agent masterbatch prepared inparagraph (1) above was added, followed by kneading at the temperaturefor 30 minutes to prepare an epoxy resin composition. Tables 1 to 4 showthe components of the epoxy resin compositions prepared in each Exampleand Comparative example.

<Method for Production of Cured Epoxy Resin>

An epoxy resin composition prepared according to the <Method forpreparation of epoxy resin compositions> described above was defoamed ina vacuum and cured at a temperature of 130° C. for 90 minutes in a moldset for a thickness of 2 mm using a 2 mm thick spacer of Teflon(registered trademark), thus providing a cured epoxy resin plate with athickness of 2 mm.

<Method for Production of Woven Fabric Carbon Fiber Reinforced CompositeMaterial (Hereinafter, Woven Fabric CFRP)>

An epoxy resin composition prepared according to the <Method forpreparation of epoxy resin composition> described above was spread on apiece of release paper using a film coater to produce a resin filmhaving a metsuke of 66 g/m². A piece of two-directional carbon fibercloth (2/2 twill weave, metsuke 198 g/m²) (Torayca (registeredtrademark) T300, manufactured by Toray Industries, Inc.) was sandwichedbetween two sheets of resin film and heat-compressed in a prepreggingapparatus to ensure impregnation from both sides to provide a wovenfabric prepreg. The resin accounted for 40 mass % of the prepreg.

Ten plies of this woven fabric prepreg were laid up with their fibersaligned in the same direction, covered with a nylon film in a gaplessmanner, and subjected to heat-compression molding in an autoclave at130° C. for 2 hours under an internal pressure of 0.3 MPa to ensurecuring to prepare a woven fabric CFRP.

<Methods for Evaluation of Physical Properties>

(1) Preservation Stability of Epoxy Resin Composition

The preservation stability of an epoxy resin composition is evaluated interms of the variation in Tg that is determined as described below. In acontainer having a round bottom with a diameter of 4 cm, 2 g of an epoxyresin composition prepared according to the <Method for preparation ofepoxy resin composition> described above was fixed and it was stored for7 days in a constant temperature and humidity tank placed in anenvironment with a temperature of 25° C. and a relative humidity of 50%RH. A 3 mg sample of the resin was weighed on a pan before and after thestorage and measurements were taken using a differential scanningcolorimeter (Q-2000, manufactured by TA Instrument) while increasing thetemperature from −50° C. to 100° C. at a constant heating rate of 10°C./minute. The midpoint in the inflection section of the resultingheat-temperature curve was taken as the glass transition temperature(hereinafter denoted as Tg). The Tg variation was calculated bysubtracting the Tg value measured before the storage from the Tg valuemeasured after the storage. A smaller Tg variation shows a betterpreservation stability.

(2) Tg of Cured Epoxy Resin

A test piece with a width of 10 mm, a length of 40 mm, and a thicknessof 2 mm was cut out of a cured epoxy resin sample prepared according tothe <Method for production of cured epoxy resin> described above andsubjected to measurement by a dynamic viscoelasticity measuringapparatus (DMA-Q800, manufactured by TA Instruments) under theconditions of a deformation mode of cantilever bending, a span of 18 mm,a strain of 20 μm, a frequency of 1 Hz, and a constant temperatureraising rate of 5° C./min for heating from 40° C. to 200° C. Tg wasdetermined as the onset temperature of storage elastic modulus in theresulting storage elastic modulus-temperature curve.

(3) Elastic Modulus of Cured Epoxy Resin

A test piece having a width of 10 mm and a length of 60 mm was cut outof a cured epoxy resin sample prepared according to the <Method forproduction of cured epoxy resin> described above and subjected tothree-point bending test using an Instron type universal tester(manufactured by Instron Corporation) with a span of 32 mm and acrosshead speed of 100 mm/minute according to JIS K7171 (1994) tomeasure the elastic modulus. Measurements were taken from 6 specimens(n=6) and the average was adopted as their elastic modulus.

(4) Yellowness Index of Cured Epoxy Resin

A 3 cm×3 cm test piece with a thickness of 2 mm was cut out of a curedepoxy resin sample prepared according to the <Method for production ofcured epoxy resin> described above. This test piece was subjected tomeasurement of the color of transmitting objects using aspectrophotometer (MSC-P, manufactured by Suga Test Instruments Co.,Ltd.) according to JIS Z8722 (2009) to determine the tristimulus values.A D65 Illuminant was used under the geometrical condition of e, andmeasurements were taken by spectrophotometric colorimetry with aneffective wavelength width of 5 nm and wavelength intervals of 5 nm. Thecolor system used was the XYZ color system. Based on the tristimulusvalues thus obtained, the yellowness index was calculated according toJIS K7373 (2006).

(5) Appearance of Woven Fabric CFRP

Woven fabric CFRP was prepared according to the <Method for productionof woven fabric CFRP> described above and immersed in 40° C. water for 7days. After the immersion, the appearance of the woven fabric CFRP wasobserved visually focusing on the portions around intersections offibers. A specimen was rated as good when no white spots were found andrated as poor when white spots were found.

Example 1

An epoxy resin composition was prepared according to the <Method forpreparation of epoxy resin composition> described above using 20 partsby mass of TEPIC (registered trademark) -L, 25 parts by mass of jER(registered trademark) 828, and 55 parts by mass of Epotohto (registeredtrademark) YDF2001 as the epoxy resins of component [A], 3 parts by massof G-8009L as the imidazole compound of component [B], and 1 part bymass of benzoic acid as the acidic compound of component [C].

For this epoxy resin composition, the Tg variation was measured andfound to be +4° C., which shows a high preservation stability.

From the epoxy resin composition obtained, cured epoxy resin wasprepared according to the <Method for production of cured epoxy resin>.The Tg, flexural modulus, and yellowness index of this cured epoxy resinwere measured and results showed a Tg of 135° C., a flexural modulus of3.5 GPa, and a yellowness index of 54, which suggest that the curedresin had good physical properties. Then, woven fabric CFRP was preparedfrom the epoxy resin composition obtained and its appearance wasobserved. The observation showed no white spots.

Examples 2 to 19

Except for using the resin components that are shown in Tables 1 and 2,the same procedure as in Example 1 was carried out to prepare epoxyresin compositions, cured epoxy resins, and woven fabric CFRPs.

The preservation stability of the epoxy resin composition, the Tg,elastic modulus, and yellowness index of the cured epoxy resins, and theappearance of the woven fabric CFRP determined in each Example are givenin Tables 1 and 2, showing that good results were obtained for all ofthem.

Example 20

Except for using a resin component free of acidic compound as shown inTable 2, the same procedure as in Example 1 was carried out to preparean epoxy resin composition, cured epoxy resin, and woven fabric CFRP.The epoxy resin composition obtained was slightly lower in preservationstability than the one in Example 1, but it was almost as good as theone in Example 1 with respect to the other physical propertiesevaluated.

Comparative Example 1

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 3. Results of evaluation of physicalproperties are also shown in Table 3. Good results were obtained for thepreservation stability of the epoxy resin composition, the Tg andelastic modulus of the cured epoxy resin, and the appearance of thewoven fabric CFRP, but resin [A2] accounted for only less than 40 partsby mass relative to the total quantity of epoxy resins, which accountedfor 100 parts by mass, and the cured epoxy resin had an undesirableyellowness index.

Comparative Example 2

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 3. Evaluation results are also shown in Table3. Good results were obtained for the Tg and elastic modulus of thecured epoxy resins and the appearance of the woven fabric CFRP, but theepoxy resin composition had a slightly low preservation stability. Inaddition, resin [A2] accounted for only less than 40 parts by massrelative to the total quantity of epoxy resins, which accounted for 100parts by mass, and the cured epoxy resin had an undesirable yellownessindex.

Comparative Example 3

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 3. Evaluation results are also shown in Table3. Good results were obtained for the preservation stability of theepoxy resin composition, the Tg and yellowness index of the cured epoxyresin, and the appearance of the woven fabric CFRP, but resin [A1]accounted for only less than 10 parts by mass relative to the totalquantity of epoxy resins, which accounted for 100 parts by mass, and thecured epoxy resin had a small elastic modulus.

Comparative Example 4

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 3. Evaluation results are also shown in Table3. Good results were obtained for the preservation stability of theepoxy resin composition, the Tg and yellowness index of the cured epoxyresin, and the appearance of the woven fabric CFRP, but resin [A1]accounted for only less than 10 parts by mass relative to the totalquantity of epoxy resins, which accounted for 100 parts by mass, and thecured epoxy resin had a small elastic modulus.

Comparative Example 5

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 3. Evaluation results are also shown in Table3. Good results were obtained for the preservation stability of theepoxy resin composition, the Tg and yellowness index of the cured epoxyresin, and the appearance of the woven fabric CFRP, but resin [A1]accounted for only less than 10 parts by mass relative to the totalquantity of epoxy resins, which accounted for 100 parts by mass, and thecured epoxy resin had a small elastic modulus.

Comparative Example 6

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 3. Evaluation results are also shown in Table3. Good results were obtained for the preservation stability of theepoxy resin composition, the Tg and elastic modulus of the cured epoxyresin, and the appearance of the woven fabric CFRP, but resin [A1]accounted for more than 40 parts by mass relative to the total quantityof epoxy resins, which accounted for 100 parts by mass, and the curedepoxy resin had an undesirable yellowness index.

Comparative Example 7

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 3. Evaluation results are also shown in Table3. Good results were obtained for the preservation stability of theepoxy resin composition, the elastic modulus and yellowness index of thecured epoxy resin, and the appearance of the woven fabric CFRP, but theaverage epoxy equivalent weight of resin [A2] failed to reach 220 g/eq,and the cured epoxy resin had a low Tg.

Comparative Example 8

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 4. Evaluation results are also shown in Table4. Good results were obtained for the preservation stability of theepoxy resin composition, the elastic modulus and yellowness index of thecured epoxy resin, and the appearance of the woven fabric CFRP, but theaverage epoxy equivalent weight of resin [A2] exceeded 500 g/eq, and thecured epoxy resin had a low Tg.

Comparative Example 9

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 4. Evaluation results are also shown in Table4. Good results were obtained for the preservation stability of theepoxy resin composition, the elastic modulus and yellowness index of thecured epoxy resin, and the appearance of the woven fabric CFRP, but thecontent of component [B] was small, and the ratio of the number ofimidazole groups to the number of epoxy groups was less than 0.01,resulting in cured epoxy resin with a low Tg.

Comparative Example 10

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 4. Evaluation results are also shown in Table4. Good results were obtained for the Tg and elastic modulus of thecured epoxy resin and the appearance of the woven fabric CFRP, but thecontent of component [B] was large and the ratio of the number ofimidazole groups to the number of epoxy groups was more than 0.06. As aresult, the epoxy resin composition had a low preservation stability andthe cured epoxy resin had an undesirable yellowness index.

Comparative Example 11

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 4. Evaluation results are also shown in Table4. Good results were obtained for the preservation stability of theepoxy resin composition, the elastic modulus and yellowness index of thecured epoxy resin, and the appearance of the woven fabric CFRP, butcomponent [B] was not contained, and the cured epoxy resin had a low Tg.

Comparative Example 12

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 4. Evaluation results are also shown in Table4. Good results were obtained for the preservation stability of theepoxy resin composition and the Tg, elastic modulus, and yellownessindex of the cured epoxy resin, but component [B] was not containedwhereas dicyandiamide was contained, resulting in white spots beingfound in the woven fabric CFRP.

Comparative Example 13

An epoxy resin composition, cured epoxy resin, and woven fabric CFRPwere prepared by the same procedure as in Example 1 using the resincomponents given in Table 4. Evaluation results are also shown in Table4. Good results were obtained for the Tg of the cured epoxy resin andthe appearance of the woven fabric CFRP, but component [A1] was notcontained, and the cured epoxy resin had a small elastic modulus. Inaddition, the content of component [B] was large and the ratio of thenumber of imidazole groups to the number of epoxy groups was more than0.06. As a result, the epoxy resin composition had an undesirablepreservation stability and the cured epoxy resin had an undesirableyellowness index.

TABLE 1 Example Example Example Example Example Components 1 2 3 4 5Epoxy resin [A] [A1] TEPIC ® -S composition epoxy resin isocyanuric acidTEPIC ® -L 20 20 10 25 40 (parts by type epoxy resin TEPIC ® -PAS B22mass) [A2] EPICLON ® 830 20 20 20 bisphenol type jER ® 828 25 15 epoxyresin Epotohto ® 55 33 YDF2001 jER ® 1001 70 55 40 jER ® 4004P jER ®1007 [A3] other jER  154 35 epoxy resin [B] [B1] G-8009L 3 3 3 3 3imidazole compound compound represented by general formula (I) [B2]Curezol ® P-0505 compound represented by general formula (II) [B3]Curezol ® 2MZ-H other imidazole Curezol ® 2PZ compound [C] [C1] benzoicacid 1 1 1 1 1 acidic aromatic compound carboxylic acid [C2] Curezol ®L-07N mixture containing borate ester compound Resin average epoxyequivalent 322 322 341 323 299 composition weight of resin [A2] g · eq⁻¹parameter average epoxy equivalent weight 224 184 276 209 168 over allepoxy resins g · eq⁻¹ ratio of imidazole groups to 0.034 0.028 0.0420.032 0.026 epoxy groups Physical epoxy resin preservation stability 4 44 4 4 properties composition (Tg variation ° C.) evaluation cured epoxyTg ° C. 135 136 125 133 144 results resin elastic modulus GPa 3.5 3.53.3 3.5 3.7 yellowness index 54 77 58 55 72 woven appearance (white spotgood good good good good fabric absent: good, CFRP white spot existent:poor) Example Example Example Example Example Components 6 7 8 9 10Epoxy resin [A] [A1] TEPIC ® -S 20 20 20 20 composition epoxy resinisocyanuric acid TEPIC ® -L (parts by type epoxy resin TEPIC ® -PAS B2220 mass) [A2] EPICLON ® 830 40 15 15 bisphenol type jER ® 828 25 25epoxy resin Epotohto ® 55 55 YDF2001 jER ® 1001 40 55 45 jER ® 4004P 10jER ® 1007 20 [A3] other jER  154 epoxy resin [B] [B1] G-8009L 3 3imidazole compound compound represented by general formula (I) [B2]Curezol ® P-0505 3 3 3 compound represented by general formula (II) [B3]Curezol ® 2MZ-H other imidazole Curezol ® 2PZ compound [C] [C1] benzoicacid 1 1 acidic aromatic compound carboxylic acid [C2] Curezol ® L-07N 11 1 mixture containing borate ester compound average epoxy equivalent322 322 253 374 417 weight of resin [A2] g · eq⁻¹ average epoxyequivalent weight 223 283 194 242 255 over all epoxy resins g · eq⁻¹ratio of imidazole groups to 0.034 0.044 0.021 0.026 0.027 epoxy groupsPhysical epoxy resin preservation stability 4 4 3 3 3 propertiescomposition (Tg variation ° C.) evaluation cured epoxy Tg ° C. 134 133128 139 127 results resin elastic modulus GPa 3.5 3.3 3.5 3.5 3.5yellowness index 63 78 54 41 34 woven appearance (white spot good goodgood good good fabric absent: good, CFRP white spot existent: poor)

TABLE 2 Example Example Example Example Example Component 11 12 13 14 15Epoxy [A] [A1] TEPIC ® -S 30 30 30 resin epoxy isocyanuric TEPIC ® -L 2020 composition resin acid type TEPIC ® (parts by epoxy resin -PAS B22mass) [A2] EPICLON ® 830 bisphenol type jER ® 828 25 25 25 25 25 epoxyresin Epotohto ® 45 45 45 55 55 YDF2001 jER ® 1001 jER ® 4004P jER ®1007 [A3] other jER ® 154 epoxy resin [B] [B1] G-8009L imidazolecompound compound represented by general formula (I) [B2] Curezol ®P-0505 2 3 8 compound represented by general formula (II) [B3] Curezol ®2MZ-H 1.3 other imidazole Curezol ® 2PZ 2.2 compound [C] [C1] benzoicacid 1 1 acidic aromatic compound carboxylic acid [C2] Curezol ® L-07N 11 1 mixture containing borate ester compound Resin average epoxyequivalent 308 308 308 322 322 composition weight of resin [A2] g · eq⁻¹average epoxy equivalent weight 190 190 190 224 224 over all epoxyresins parameter g · eq⁻¹ ratio of imidazole groups 0.014 0.020 0.0540.036 0.034 to epoxy groups Physical epoxy resin preservation stability0 3 7 4 4 properties composition (Tg variation ° C.) evaluation cured Tg° C. 128 137 148 130 132 results epoxy elastic modulus GPa 3.5 3.5 3.53.5 3.5 resin yellowness index 27 41 70 62 67 woven appearance (whitespot good good good good good fabric absent: good, CFRP white spotexistent: poor) Example Example Example Example Example Component 16 1718 19 20 Epoxy [A] [A1] TEPIC ® -S resin epoxy isocyanuric TEPIC ® -L 2020 20 20 20 composition resin acid type TEPIC ® -PAS (parts by epoxyresin B22 mass) [A2] EPICLON ® 830 bisphenol type jER ® 828 25 25 25 2525 epoxy resin Epotohto ® 55 55 55 55 55 YDF2001 jER ® 1001 jER ® 4004PjER ® 1007 [A3] other jER ® 154 epoxy resin [B] [B1] G-8009L 3 imidazolecompound compound represented by general formula (I) [B2] Curezol ®P-0505 4.3 3 3 3 compound represented by general formula (II) [B3]Curezol ® 2MZ-H other imidazole Curezol ® 2PZ compound [C] [C1] benzoicacid 1 acidic aromatic compound carboxylic acid [C2] mixture Curezol ®L-07N 0.5 3 6 containing borate ester compound Resin average epoxyequivalent 322 322 322 322 322 composition weight of resin [A2] g · eq⁻¹average epoxy equivalent weight 224 224 224 224 224 over all epoxyresins parameter g · eq⁻¹ ratio of imidazole groups 0.034 0.034 0.0340.034 0.034 to epoxy groups Physical epoxy resin preservation stability4 5 3 1 11 properties composition (Tg variation ° C.) evaluation curedTg ° C. 138 136 133 131 135 results epoxy elastic modulus GPa 3.5 3.53.5 3.5 3.5 resin yellowness index 40 52 56 51 59 woven appearance(white spot good good good good good fabric absent: good, CFRP whitespot existent: poor)

TABLE 3 Compar- Compar- Compar- Compar- Compar- Compar- Compar- ativeative ative ative ative ative ative example example example exampleexample example example Components 1 2 3 4 5 6 7 Epoxy [A] [A1] TEPIC ®-L 20 20 50 20 resin epoxy isocyanuric compo- resin acid type sitionepoxy resin (parts by [A2] EPICLON ® 20 80 mass) bisphenol 830 typejER ® 828 10 10 31 25 25 epoxy Epotohto ® 22 22 69 55 55 resin YDF2001jER ® 1001 30 jER ® 1007 [A3] SUMI- 20 other EPOXY ® epoxy ELM 434 resinJER ® 154 48 48 20 [B] [B2] Curezol ® 3 3 3 3 3 3 3 imida- compoundP-0505 zole represented com- by general pound formula (II) [B3]Curezol ® other 1B2P imidazole compound [B’] JER Cure ® hardener DICY7other than DCMU99 imidazole compound [C] [C2] Curezol ® acidic mixtureL-01B com- containing Curezol ® 1 1 1 1 1 1 pound borate ester L-07Ncompound Resin average epoxy equivalent 322 322 323 322 322 279 172compo- weight of resin [A2] g · eq⁻¹ sition average epoxy equivalentweight 176 176 323 241 277 148 151 para- over all epoxy resins meterparameter g · eq⁻¹ ratio of imidazole groups 0.019 0.019 0.035 0.0260.030 0.016 0.016 to epoxy groups Character- epoxy preservationstability 4 10 4 4 4 4 4 istics resin (Tg variation ° C.) evalu- compo-Tg ° C. 138 137 129 127 127 137 112 ation sition elastic modulus GPa 3.53.5 3.0 3.1 3.1 3.7 3.5 results cured yellowness index 100 98 26 79 61101 68 epoxy appearance good good good good good good good resin (whitespot absent: woven good, white fabric spot existent: poor) CFRP

TABLE 4 Compar- Compar- Compar- Compar- Compar- Compar- ative ativeative ative ative ative Components example 8 example 9 example 10example 11 example 12 example 13 Epoxy [A] [A1] TEPIC ® -L 20 20 20 2020 resin epoxy isocyanuric composition resin acid type (parts by epoxyresin mass) [A2] EPICLON ® 830 20 25 25 25 25 bisphenol type jER ® 82870 epoxy resin Epotohto ® YDF2001 jER ® 1001 55 55 55 55 jER ® 1007 60[A3] SUMI-EPOXY ® other epoxy ELM434 resin JER ® 154 30 [B] [B2]Curezol ® P-0505 3 1 8 20 imidazole compound compound represented bygeneral formula (II) [B3] JER ® 1B2PZ other imidazole compound [B’] JERCure ® DICY7 6 hardener DCMU99 6 3 other than imidazole compound [C][C2] Curezol ® L-01B 11 acidic mixture Curezol ® L-07N 1 1 1 compoundcontaining borate ester compound Resin average epoxy equivalent 545 306306 306 306 189 compo- weight of resin [A2] g · eq⁻¹ sition averageepoxy equivalent weight 290 218 218 218 218 186 parameter over all epoxyresins g · eq⁻¹ ratio of imidazole groups 0.031 0.008 0.062 — — 0.133 toepoxy groups Character- epoxy preservation stability 4 1 11 4 4 10istics resin (Tg variation ° C.) evalu- compo- Tg ° C. 111 100 149 105124 143 ation sition elastic modulus GPa 3.5 3.5 3.5 3.5 3.5 3.1 resultscured yellowness index 27 29 126 73 30 151 epoxy appearance good goodgood good poor good resin (white spot woven absent: good, white fabricspot existent: poor) CFRP

The epoxy resin composition according to the present invention serves toproduce a cured epoxy resin that simultaneously realizes a high heatresistance, a high elastic modulus, and a low color and therefore, fiberreinforced composite materials containing this as matrix resin have ahigh heat resistance, good mechanical properties, and a low color.Furthermore, molded articles produced from such fiber reinforcedcomposite materials do not suffer the formation of white spots on thesurface thereof, and this feature, in combined with their low colorfeature, can ensure high designability. The epoxy resin composition,prepreg, and fiber reinforced composite material according to thepresent invention can be applied favorably to sport applications andgeneral industrial applications.

The invention claimed is:
 1. A prepreg comprising an epoxy resincomposition and carbon fiber as reinforcing fiber, wherein the epoxyresin composition comprises an epoxy resin as component [A] and animidazole compound as component [B] and meeting the undermentionedconditions (a) to (d): (a) the epoxy resin composition contains anisocyanurate type epoxy resin [A1] as component [A] in an amount of 10to 30 parts by mass relative to the total quantity of epoxy resins whichrepresents 100 parts by mass, (b) the epoxy resin composition contains abisphenol type epoxy resin [A2] as a component [A] in an amount of 40 to90 parts by mass relative to the total quantity of epoxy resins whichrepresents 100 parts by mass, (c) resin [A2] has an average epoxyequivalent weight of 220 to 500 g/eq, and (d) the content of component[B] is such that the ratio of the number of imidazole groups to thetotal number of epoxy groups in the epoxy resins is 0.01 to 0.06,wherein the imidazole compound as component [B] is present as a hardenerto promote self-polymerization of the epoxy resins in component [A], andwherein the imidazole compound is used as a curing agent.
 2. A prepregas set forth in claim 1 wherein component [B] has an imidazoleequivalent weight of 180 g/eq or more.
 3. A prepreg as set forth inclaim 1 wherein a compound [B1] as represented by general formula (I) iscontained as component [B]:

wherein R₁, R₂, R₃ and R₄ are each independently a hydrogen atom, analiphatic hydrocarbon group with a carbon number of 1 to 20, or a phenylgroup, and X is an alkylene group or an aromatic hydrocarbon group.
 4. Aprepreg as set forth in claim 1 wherein a compound [B2] as representedby general formula (II) is contained as component [B]:

wherein R₅, R₆, R₇ and R₈ are each independently a hydrogen atom, analiphatic hydrocarbon group with a carbon number of 1 to 20, or a phenylgroup, and Y is a single bond, an alkylene group, an alkylidene group,ether group, or sulfonyl group.
 5. A prepreg as set forth in claim 1further comprising an acidic compound as component [C].
 6. A prepreg asset forth in claim 5 comprising an aromatic carboxylic acid [C1] ascomponent [C].
 7. A prepreg as set forth in claim 5 comprising a borateester compound [C2] as component [C].
 8. A prepreg as set forth in claim1 wherein dicyandiamide accounts for 0.5 part by mass or less relativeto the total quantity of epoxy resins which accounts for 100 parts bymass.
 9. A prepreg as set forth in claim 1 wherein the reinforcing fiberis in the form of woven fabric.
 10. A fiber reinforced compositematerial obtainable by curing a prepreg as set forth in claim
 1. 11. Aprepreg as set forth in claim 1 wherein a cured product of the epoxyresin has a Yellowness Index of 78 or less.
 12. A prepreg as set forthin claim 1 wherein the epoxy resin as component [A] consist of theisocyanurate type epoxy resin [A1] and the bisphenol type epoxy resin[A2].